1. Field of the Invention
This invention relates to an optical inspection system and more particularly to an improved system for rapidly determining the location and size of via openings in substrate sheets.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, via openings are formed in ceramic substrate sheets in order to provide connecting paths through the sheets. A number of sheets are used to form a multi-layer module that mechanically supports and electrically interconnects a plurality of integrated circuit chips mounted on the module. The via openings are very small and are usually closely spaced (e.g., 100 microns in diameter and 600 microns spacing). The ceramic substrates are sintered after the vias are formed. This sintering process can sometimes cause a change in the location of a via from its intended location; the difference between the intended location and the actual location is called distortion. For the next generation of substrates, which utilize a thin-film process, this distortion can affect the via locations; thereby preventing a satisfactory electrical connection between the via conductor and the thin-film wiring. It can also affect the electrical connection between two or more substrate sheets.
There are several prior art systems commercially available for measuring via distortion. They use an area array CCD detector and accompanying optics to illuminate the surface of the ceramic substrate and reflect light from the surface to the area array. A typical system of this type moves the substrate (under servo control) to a series of programmed positions relative to CCD area array. An image of a via is acquired by the CCD area array while the substrate is stationary. Because of the time required to position the substrate and to form the via image in prior art inspection systems, vias are inspected for distortion on a sampling basis only; usually only about one percent of the vias are inspected. Inspection by continuous scanning of the substrate surface has heretofore not been possible because of inadequate light available at the scanned surface to provide sufficient contrast between the via opening and the surface itself, and because an area CCD array is unsuited to continuous scanning.